1. Field of the Invention
This invention relates to an improved apparatus and process for positioning an implement, such as a welding head, relative to a workpiece using visual sensing. More particularly, it relates to such an apparatus and process suitable for real time automatic control of arc welding, in which a variety of different shaped workpieces are to be welded.
2. Description of the Prior Art
It has been recognized for some time that it would be desirable to provide an automated arc welding apparatus and process. There is a large and growing shortage of skilled welding operators. This shortage is aggravated by the poor working environment in most arc welding installations, and the consequent high rate of personnel turnover. There is also an increasing government and union pressure to remove people from dangerous or potentially unhealthful environments.
In addition to the above factors, there is a continuing and growing need to decrease direct labor costs incurred in the manufacture of arc-welded goods. The significance of this need is highlighted by the fact that labor and associated overhead charges account for more than 80% of the direct costs per unit length of weld.
Further, there is an increasing use in automotive and other industries of materials that are very sensitive to weld defects. Such materials require more consistent weld quality than can be provided by a human operator.
While a variety of approaches has been proposed for automatic positioning of weld heads, it should be recognized that, in many manual welding operations, the human operator functions not only as a manipulator, but also as a real time sensing and feedback medium, making small adjustments in welding parameters and procedure to accommodate perceived changes in joint orientation, dimensions and fit-up. To some extent, this accommodation can be provided by specially designed hard tooling or by a preprogrammed positioner. In many manufacturing situations, however, this approach is not feasible due to the inherent inaccuracy of the forming and cutting processes used in workpiece production and by other unpredictable thermal and mechanical effects. It is therefore necessary to equip an automated welding system with the capability of acquiring and analyzing sensory data in real time. This enables the welding apparatus to follow the joint in space, and at the same time adjust welding parameters, such as wire feed and travel speeds, torch attitude and arc voltage. Real time data acquisition and analysis further would allow an automatic welding system to select among alternative welding procedures, such as "stringering", i.e., weaving to bridge fit-up gaps, to accommodate variations in joint type, geometry, location, and fit-up.
Presently available sensors for arc welding fall into three general categories. The first category utilizes some form of mechanical or tactile contact devices, such as electromechanical probes and various forms of guide rollers and pins. The second category utilizes eddy current sensing devices. The third category employs feedback from such welding parameters as arc voltage or current waveforms to control the welding.
Such systems suffer from a number of disadvantages. Both mechanical and eddy current devices must be mechanically reconfigured for different joint types, such as butt joints and fillet joints. Further, mechanical and eddy current sensors must either contact the workpiece or be in close proximity to it during operation. Contact between a workpiece and a sensing element will produce mechanical wear. Maintaining a non-cntact sensor in close proximity to the workpiece creates an increased probability of mechanical damage to the sensing element.
Recognizing the problems associated with the above three types of welding control systems, various proposals have been made in the prior art for visual determination of welding workpiece geometry. For example, U.S. Pat. No. 3,976,382 discloses a system in which a light source is used in combination with a light shield positioned close to the workpiece to cast a shadow of known predetermined shape on the workpiece. Variation of the shadow from the known shape then provides a basis for characterizing the shape of the workpiece. However, that technique requires the shield to be positioned close to the workpiece. The shadow technique is therefore subject to many of the same disadvantages as the eddy current technique described above.
Another welding head positioning system which utilizes a visual sensor is disclosed by T. Nozaki, et al., "Robot `Sees`, Decides and Acts", Welding and Metals Fabrication, Vol. 47, No. 9, pp. 647-658 (November, 1979). However, that system requires user entry in use of the system of certain welding geometry information, and it is limited in the types of welds that can be accommodated, because of the limited information on weld geometry that can be obtained with the visual sensor there disclosed.
Thus, while a substantial amount of work has already been done on automatic positioning apparatus for weld heads and similar implements, a need still remains for further development of an automatic apparatus for positioning an implement relative to a workpiece, which will meet the stringent demands of welding processes on such apparatus.